1. Field of the Invention
This invention relates to a magnet roll and method for manufacturing the same, and particularly to a magnet roll suitable for use in a developing device of electrophotographic copier as disposed inside of a developing sleeve along the circumference of which is transported toner.
2. Description of the Prior Art
In a developing device for developing an electrostatic latent image formed on an imaging surface, such as the surface of an electrophotographic member, by application of toner to the latent image, a developing sleeve is rotatably provided as a toner carrier and a magnet roll is disposed inside of the sleeve thereby producing a magnetic field at the surface of the sleeve to have the toner magnetically attracted to the peripheral surface of the sleeve. Such a magnet roll typically includes a magnet formed by sintering a magnetic material such as ferrite. In this case, however, since ferrite is brittle, there is difficulty in forming various shapes using ferrite and ferrite must be provided at those locations where unnecessary so as to maintain its integrity when manufactured. For this reason, the prior art magnet roll tended to be heavier in weight and higher in manufacturing cost thereby requiring the provision of costly associated parts for supporting and driving to rotate the magnet roll. Moreover, since the prior art magnet roll was manufactured by sintering, there was a difficulty in dimensional control due to distortions caused by sintering, which could also hinder to attain an intended performance. It is true that a secondary processing may be carried out to the magnet roll after sintering to attain a desired dimensional accuracy, but such a secondary processing will push up the manufacturing cost.
FIG. 1 shows a prior art doughnut-shaped magnet roll 1. In this case, the doughnut-shaped magnet 1a was first formed by sintering and then fitted onto and fixed to a shaft 1b, for example, by an appropriate adhesive. In this example, the outer peripheral surface of the magnet 1a required to be abraded as a secondary step so that there was a difficulty in manufacture. Besides, the overall structure was quite heavy and thus it required a large amount of driving power for rotation.
FIG. 2 shows another prior art magnet roll 2 which is suited for providing a plurality of magnetic poles at unequal spacing as different from the structure of FIG. 1. In this case also, the overall structure is doughnut-shaped with intermediate portions A interposed between adjacent magnetic portions 2a in order to increase integrity of the structure. This magnet roll is also disadvantageous because the portions A must be provided to compensate the physical weakness of ferrite magnets 2a thereby increasing not only weight but also cost.
FIG. 3 shows a further prior art magnet roll 3 which was proposed to overcome the shortcomings of the previous two prior art magnet rolls shown in FIGS. 1 and 2. That is, in this case, in order to make the overall structure light in weight by removing unnecessary portions, a plurality of elongated, rectangularly shaped magnets 3a were first manufactured and fixedly attached to a cylindrical support 4 at those locations where required. In this case, it is true that the overall weight may be minimized, but the manufacturing steps are increased because a plurality of magnets 3a must be fixedly attached to the support 4 one after another. Moreover, the shape of support 4 is rather complicated because of required positioning and secure holding of the individual magnets 3a, so that there is produced another difficulty in forming the support 4. On the other hand, since the magnet 3a is rectangular in cross section, the largest gap G is formed between the magnet 3a and the inner peripheral surface of a developing sleeve 5 at the midpoint G1 of the magnet 3a where the largest magnetic force is normally required when assembled, as shown in FIG. 4. Thus, the structure of FIG. 3 is disadvantageous also from the viewpoint of performance. It is true that one side of the magnet 3a may be formed into a shape to be in compliance with the inner peripheral surface of the sleeve 5; however, such a secondary processing can be a cause of pushing up the manufacturing cost.